A horizontal rotary hook providing an easy replacement of a bobbin has conventionally been used in sewing machines. The horizontal rotary hook comprises an outer rotating hook having a sliding surface and an inner bobbin case holder accommodating a thread bobbin therein and supported in the rotating hook so as to be rotatable relative to the sliding surface. The rotating hook is formed into a cylindrical container with an upper opening. The rotating hook includes a peripheral wall on which a loop seizing beak is provided for seizing a loop of needle thread and which has a needle-thread passing opening through which a needle thread seized by the beak passes. The rotating hook is rotated in a predetermined direction by a sewing machine motor.
The bobbin case holder is made of a synthetic resin so that a reduction is achieved in the weight and production cost thereof. Furthermore, the rotating hook has also been made of a synthetic resin recently. For example, JP-U-S60-149388 discloses a horizontal rotary hook in which a beak member necessitating durability is made of a metal and mounted on the rotating hook so as to be laid on an inner surface of the peripheral wall of the rotating hook. Furthermore, JP-A-S58-97390 discloses a horizontal rotary hook in which a separate hook beak and a separate needle guard (a beak member) are disposed in an opening defined in the peripheral wall of the rotating hook to be mounted to the peripheral wall by small screws. The document discloses nothing about a material for the rotating hook.
However, the peripheral wall of the rotating hook is made of a synthetic resin in the horizontal rotary hook disclosed by the former reference, JP-U-S60-149388. Accordingly, an end of the rotating hook defining the needle-thread passing opening particularly has a low strength, so that there is a possibility that the end may be deformed or broken. On the other hand, in the horizontal rotary hook disclosed by the latter reference, JP-A-S58-97390, a part of the peripheral wall near the opening has an excessively large thickness, whereupon a predetermined strength is ensured. However, this results in another problem that a size of the rotating hook is increased.